G4ALG's QRP Radio Pages

[ Previously GW4ALG (QRT in February 2007) ]

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Experimental Valve Transmitter

This page describes an abandoned project, but the details are retained here for reference.

The objective was to make a broadband two-valve transmitter based on Mullard QQV02-6 and QQV03-10 valves.  However, the lack of tuned (tank) circuits resulted in low gain stages that struggled to provide a useful output power.

I was attempting to make a broadband valve amplifier with a QQV02-6 driving a QQV02-6 or QQV03-10 in the PA having a 50 ohm input impedance delivering 5 watts into 50 ohms, covering the range 1.8 to 14 MHz.  I sought to do this without using any tuned circuits.  My desire to operate several bands using one unit, with no band switching, justified this experiment.

Of course, switched low pass filters would have been required between the transmitter and the antenna. 

Here's a photo of the first prototype, including the prototype power supply:

Using the prototype, with a QQV02-6 driving a QQV02-6, I managed to get 1 watt of RF out on several bands using a commercial transceiver as the VFO source.  The low pass filter module came from an FT767GX and was purchased via eBay.  

My first QSO was with John, M0CVB near Taunton on 5 MHz.  My second QSO was with Stefan DF7CP on 3.5 MHz, who was also using a home made valve TX!

I was very pleased to work Tom, GM3MXN for my third QSO on 5MHz using this transmitter.  Tom is a very experienced QRPer, sends lovely morse, and had no trouble copying my tentative 1 watt signal, sending me a 559 report.   I've also managed a QSO on 7 MHz, so things are looking promising!

In the end, I couldn't get more than 1 watt RF from this setup, even if I substantially increased the drive. 

Here's the circuit.  Not shown is the low pass filter module (between the TX and the antenna).  A datasheet for the QQV02-6 can be found here.


The circuit of the prototype power supply built specially to power the transmitter is shown below.  Because I no longer have any HT transformers, I used two 30 volt transformers back to back to obtain the HT voltages.  Note that not all the voltage outputs have been used in this project.  The 15 VDC rail was used to power a 12 volt regulator to operate the relays in the switched low pass filter.